DC-DC converters that can be used for both step-up conversion and step-up/down conversion are conventionally known. FIG. 8 shows the circuit configuration of a typical example. This DC-DC converter has choke coil 100 used as the inductance element. An input feedforward converter 102 is connected between voltage input terminal IN, which receives a DC input voltage Vin from an external battery BT, and node Nx, one of the terminals of choke coil 100. An output feedback control booster circuit 104 is connected between the other terminal of choke coil 100 and voltage output terminal OUT. Said booster circuit 104 has a boosting function, which can boost voltage Vx applied from feedforward converter 102 to node Nx (referred to as “node voltage Vx” hereinafter) and generate an output voltage Vout of the desired level to voltage output terminal OUT. Consequently, feedforward converter 102 must output node voltage Vx lower than the desired level of output voltage Vout independent of the level of input voltage Vin to node Nx.
Feedforward converter 102 is comprised of switching element 106 connected between voltage input terminal IN and node Nx, switching element 108 connected between node Nx and the ground potential, and feedforward control circuit 110, which turns on/off or switches said switching elements 106 and 108 in a complementary manner corresponding to the input voltage Vin input to voltage input terminal IN. Transistors, for example, NMOS transistors are usually used for said switching elements 106 and 108. Feedforward control circuit 110 has input voltage monitoring circuit 112, PWM (pulse width modulation) comparator 114, and inverter 116. In this case, input voltage monitoring circuit 112 has resistance voltage-dividing circuit 118, operational amplifier 120, feedback resistor 122, and reference voltage generating circuit 124.
In input voltage monitoring circuit 112, a coefficient −Ka (Ka>0) determined by the voltage-dividing ratio of resistance voltage-dividing circuit 118 and the resistance of feedback resistor 122 is multiplied to input voltage Vin, and a constant C (C>0) corresponding to reference voltage Vref sent from reference voltage generating circuit 124 is added to the multiplication result. A DC voltage Vf (referred to as “feedforward voltage Vf” hereinafter) corresponding to the final calculation result is obtained. In other words, feedforward voltage Vf can be expressed by the following equation (1).Vf=−KaVin+C  (1)
Consequently, as shown in FIG. 9, as the level of input voltage Vin is increased, feedforward voltage Vf becomes lower in reverse proportion to the input voltage. The feedforward voltage Vf sent from input voltage monitoring circuit 112 is input to input terminal (−) of comparator 114. On the other hand, a triangular signal Wa having a prescribed frequency and a prescribed voltage level or peak level (peak-to-peak value) is input from a triangular signal generating circuit (not shown in the figure) to input terminal (+) of comparator 114.
Comparator 114 compares the level of feedforward voltage Vf and the voltage level of triangular signal Wa. When Vf>Wa, control signal Sa with an output voltage level L is output. When Vf<Wa, control signal Sa with level H is output. Control signal Sa is applied to the control terminal of switching element 108. Control signal Sb, obtained by inverting Sa using inverter 116, is applied to the control terminal of switching element 106.
In feedforward converter 102, as shown in FIGS. 9 and 10, when input voltage Vin is equal to set level Vs, feedforward voltage Vf is set to be almost equal to the maximum peak level of triangular signal Wa. In this case, set level Vs is set to be much lower than output voltage Vout (desired level).
When input voltage Vin is lower than set level Vs, the relationship of Vf>Wa is maintained, and comparator 114 keeps the output on level L. Control signals Sa and Sb are kept to Sa=level L and Sb=level H, switching element 106 is kept in the on state, switching element 108 is kept in the off state. As a result, the input voltage Vin from voltage input terminal IN is sent to node Nx via switching element 106 in the on state, and node voltage Vx is almost equal to input voltage Vin is at node Nx. Booster circuit 104 receives node voltage Vx(=Vin) from node Nx via choke coil 100 and outputs generates voltage Vout with the desired level to output terminal OUT as a result of a feedback controlled boosting operation. As described above, input voltage Vin is extracted through feedforward converter 102 and is then applied to booster circuit 104 via choke coil 100. A boosting operation is performed by the entire DC-DC converter.
When input Vin is higher than set level Vs, as shown in FIG. 10, feedforward voltage Vf crosses with triangular signal Wa, and one cycle of triangular signal Wa is divided into the period of Vf<Wa and the period of Vf>Wa. During the period of Vf<Wa, Sa=level H and Sb=level L on the output side of comparator 114. Switching element 108 is turned on, and switching element 106 is turned off. On the other hand, during the period of Vf>Wa, Sa=level L, Sb=level H on the output side of comparator 114. Switching element 106 is turned on, and switching element 108 is turned off. Voltage Ex, obtained by averaging node voltage Vx obtained at node Nx over time (referred to as “pseudo input voltage Ex” hereinafter), can be expressed by equation (2) as follows, where d is the duty cycle during the period when switching element 106 is on during one cycle of triangular signal Wa.Ex=d·Vin  (2)
As described above, when input voltage Vin is higher than set level Vs, input voltage Vin is reduced by feedforward converter 102 to the level of pseudo input voltage Ex corresponding to the duty cycle d and is then applied to booster circuit 104 via choke coil 100. A step-up/down conversion is performed by the entire DC-DC converter.
In the aforementioned conventional DC-DC converter, the duty cycle d in feedforward converter 102 is fixed at d=1 (100%) in the step-up mode. On the other hand, in the step-up/down mode, as can be seen from FIGS. 9 and 10, the duty cycle varies linearly in reverse proportion to input voltage Vin. In other words, as shown in FIG. 10, the duty cycle d in the step-up/down mode can be expressed as a primary function of feedforward voltage Vf as shown in the following equation (3).d=1−(Vfs−Vf)/(Vfs−Vfe)  (3)
In this case, Vfs is the level of Vf when it equals the maximum peak level of triangular signal Wa, and Vfe is the level of Vf when it equals the minimum peak level of triangular signal Wa.
Based on said equations (1) and (3), duty cycle d can be expressed as a linear function of input voltage Vin as shown in equation (4) below.d=−A·Vin+B  (4)wherein, A=Ka/(Vfs−Vfe) and B=(C−Vfe)/(Vfs−Vfe).
Based on said equations (2) and (4), pseudo input voltage Ex at node Nx can be expressed as a second degree of input voltage Vin as shown in equation (5) below.Ex=−A·Vin2+BVin  (5)
FIG. 11 shows the relationship between input voltage Vin and pseudo input voltage Ex. As described above, when input voltage Vin is lower than set level Vs, Ex=Vin in the step-up mode. When input voltage Vin is higher than set level Vs, said equation (5) becomes valid in the step-up/down mode. Set level Vs can be selected at will as Vs1, Vs2, Vs3 shown in FIG. 11 by adjusting the voltage-dividing ratio of resistance voltage-dividing circuit 118.
In FIG. 11, VL is the upper limit for pseudo input voltage Ex. When pseudo input voltage Ex exceeds upper limit VL, the boosting margin (Vout−VL) on the side of booster circuit 104 cannot be guaranteed. Also, the output voltage Vout exceeds the nominal level, the regulation of booster circuit 104 becomes ineffective, and the transient response becomes poor. Consequently, it is necessary to select set level Vs with a margin so that pseudo input voltage Ex will not exceed upper limit VL. However, as shown in FIG. 12, as the set level Vs is reduced, efficiency φ in the step-up/down mode is also decreased. In this case, efficiency φ is the result of dividing the output power by the effective input power (expressed as a percentage). The characteristic curves g1, g2, g3 in FIG. 12 correspond to the characteristic curves G1, G2, G3 in FIG. 11, respectively. Since switching loss (power loss) occurs in feedforward converter 102 in the step-up/down mode, the efficiency tends to drop more than that in the step-up mode. This loss of efficiency is undesired. Consequently, although selection of the set level Vs1 to obtain the characteristic curve G1 shown in FIG. 11 is relatively optimum, it is not good enough (not absolutely optimum).
Also, in the aforementioned conventional DC-DC converter, ringing occurs in the output voltage as a result of switching between the step-up mode and the step-up/down mode. FIG. 13 shows a simulation example. The cause of the ringing is explained below.
Usually, the input voltage Vin of battery BT is input to voltage input terminal IN via a power supply line (wiring) on the circuit substrate. At any time, a voltage drop δV in proportion to the product of the impedance (intrinsic value) of the power supply line and input current Iin occurs. Consequently, (VBT−δV), obtained by subtracting the voltage drop δV on the power supply line from output voltage VBT of battery BT, is input as input voltage to voltage Vin input terminal IN.
Now, the output voltage VBT of battery BT rises, for example, in the charging mode, and the input voltage Vin input to voltage input terminal IN exceeds the set level Vs. As described above, this DC-DC converter operates in the step-up mode when Vin<Vs and switches to the step-up/down mode when Vin>Vs. However, since efficiency φ drops significantly as shown in FIG. 12 when switching to the step-up/down mode, the consumed current or input current lin increases stepwise. When that occurs, the voltage drop δV on the power supply line increases stepwise. The input voltage in (VB,−δV) returns to the relationship of Vin<Vs, and the DC-DC converter switches from the step-up/down mode to the step-up mode. However, no matter whether the converter returns to the step-up mode or not, efficiency φ increases stepwise, and input current Iin decreases stepwise. As a result, the voltage drop δV on the power supply line also decreases stepwise, and input voltage Vin(VBT−δV) rises again. Then, the DC-DC converter switches from the step-up mode to step-up/down mode again when Vin>Vs. The aforementioned operation is then repeated, and the converter switches between the step-up mode and step-up/down mode. Therefore, undesired ringing occurs in the voltage of each part of the converter. The ringing occurring in the output voltage Vout at voltage output terminal OUT will significantly deteriorate the quality and reliability of the power supply voltage to the load. Such ringing is undesirable. When the output voltage VBT of battery BT drops and the converter switches from the step-up/down mode to the step-up mode, ringing also occurs as described in the voltage nodes. When the change (rise or drop) of battery voltage VBT is alleviated, the ringing phenomenon continues during the mode transition and can lead to an oscillating state.